1. Field of the Invention
The present invention relates generally to a secondary lithium battery, and particularly to a secondary battery containing an organic electrolyte.
2. Prior Art
Secondary lithium batteries have a much higher energy than conventional batteries, such as lead-acid or Ni-Cd batteries, because of a high electrode potential and the light weight of the lithium. Several kinds of small coin-type lithium secondary batteries have already been commercialized. However, secondary lithium batteries of a larger size, such as AAA to D size, are not readily available yet, mainly due to two reasons. The first reason is the poor rechargeability of lithium in an organic electrolyte. In other words, the charge-discharge cycling efficiency of lithium is low. The second reason relates to safety problems associated with the larger size cell. The large size cell may generate a lot of gases, may catch fire or in extreme cases, may explode under some cycling conditions such as extremely high current discharge or heating. There is a strong correlation between both of the problems enumerated above and the combination of electrolyte, anode and cathode materials.
Although lithium can be used to make a high energy battery, the high activity of lithium can also make the battery unsafe. Lithium reacts with organic solvents thermodynamically to form films on the surface of the lithium, usually accompanied with gas generation as by-product. This is an exothermic reaction which can lead to the melting of the lithium which has a low melting point (180.degree. C.). Once the lithium has melted, it can react directly and vigorously with the cathode and electrolyte, producing more heat inside the battery and further accelerating these reactions. Also, some decomposition products or electrolyte itself may be easily burnt. After cycling, the battery is less safe. The morphology for lithium deposition worsens with each cycle, increasing the surface area of the highly reactive deposited Li. Therefore, by improving Li cycling efficiency by making the lithium electrochemically active, one is faced by the drawback of concomitantly making the battery unsafe.
Thus, in order to have a practical application of an electrolyte for a secondary lithium battery, both Li cycling efficiency and cell safety should be considered and satisfied. To realize this, the electrolyte materials and the composition should be optimized to each cathode.
Many kinds of organic electrolytes have been proposed to improve lithium cycling efficiency, although the safety of these are seldom demonstrated or addressed in the art. Up to now, the use of 2-methyltetrahydrofuran (2MeTHF) as a component of the electrolyte appeared to be promising in terms of improving the lithium cycling efficiency. For example, 2MeTHF-single solvent electrolyte has been proposed, based on the lithium cycling efficiency measurements by using a lithium-half-cell [V.R.Koch, U.S. Pat. No. 4,118,550 (1978)]. The addition of 2-methylfuran (2MeF) to 2MeTHF or a mixture of 2MeTHF and ether, such as nonsubstituted THF, has also been suggested [K. H. Abraham, J. S. Foos and S. B. Brummer, U.S. Pat. No. 4,489,145 (1988)] based on the cycling results of TiS2/Li cells. The addition of ethylene carbonate (EC) to 2MeTHF has also been proposed. For example, EC/2MeTHF-binary mixed solvent systems show better cycleability for V205/Li cells, where at least more than 50% 2MeTHF in volume is necessary to obtain the highest lithium cycling efficiency [S. Tobishima, M. Arakawa, T. Hirai and J. Yamaki, U.S. Pat. No. 4,737,424 (1988)]. Also, EC/2MeTHF (10/90) is recommended for TiS2/Li cells to improve Li cycling efficiency [D. H. Shen, S. Subbarao, F. Deligiannis, C. K. Huang and G. Helpert, proceeding on the symposium on `Rechargeable Lithium Batteries`, proceedings volume 90-5, The Electrochemical Society Inc., pp. 114-126, (1990)]. The addition of propylene carbonate (PC), which has a similar chemical structure to EC, to 2MeTHF was examined by Li-half-cell cycling tests [S. Tobishima and A. Yamaji, Electrochim. Acta., vol. 29, No. 10, pp. 1471-1476 (1984)]. The effects of PC addition on Li cycling efficiency was much smaller than that of EC. Also, EC/PC/2MeTHF in a preferred ratio of 12.5/12.5/75 has been disposed for a VO2/Li cell [J. R. Dahn, T. V. Buuren and U. Vonsacken, U.S. Pat. No. 4,965,150 (1990). In all of the cases mentioned above relating to 2MeTHF-based electrolytes, at least more than 50% 2MeTHF content is necessary to obtain the longest cycle life. However, the Applicant has found that a high 2MeTHF content causes a major problem from a practical point of view. 2MeTHF is easily flammable, has a flash point of -11.degree. C. and is volatile. It has a boiling point of about 80.degree. C. under 760 Torr of pressure and easily oxidizes to produce the explosive peroxide. From a practical standpoint, while the use of a large amount of 2MeTHF Will increase the cycling efficiency of the lithium, it will also make the cell unsafe. Therefore, it is essential to optimize the amount of 2MeTHF to be used for each cathode chemistry.
An EC/PC (ester/ester) mixed solvent system has been disclosed for a rechargeable MoS2/Li cell [F. C. Laman and K. Brandt, J. Power Sources, vol. 24, pp. 195-206 (1988)]. EC and PC have a high boiling point and flash point. Although for V205/Li cell and Li-half-cell cycling, EC/PC shows worse Li cycling efficiency than EC/2MeTHF, EC/PC may be safer than ether-electrolytes (S. Tobishima, M. Arakawa, T. Hirai and J. Yamaki, J. Power Sources, vol. 20, pp. 293-297, (1987)). In addition, to improve Li cycling efficiency and safety of the NbSe3/Li battery, the effects of addition of glymes to EC/PC has been reported [L.E. Brand, U.S. Pat. No. 4,753,859 (1988)]. However, suitable electrolyte material and composition are required to be determined in regard to both cycle life and safety.
Many kinds of cathode active materials have also been proposed for secondary lithium battery. Among them, vanadium oxide-based materials are promising in regard to energy and cycle life. For example, crystalline and amorphorous V205 and V6013 have a high energy and a long cycle life. Also, lithiated vanadium oxides, such as Li .alpha. V308 (1.0=&lt;.alpha.=&lt;1.2), are known. As a suitable electrolyte system for the cell with vanadium oxide as a cathode active material, EC/2MeTHF mixed solvent electrolytes have been proposed for a long cycle life [S. Tobishima, M. Arakawa, T. Hirai and J. Yamaki, J.Power Sources, vol.20, pp. 293-297, (1987)]. However, this electrolyte from the practical standpoint, has the drawback as to safety mentioned above.
Accordingly, it is an object of the invention to provide an electrolyte for use in a secondary lithium battery, which has good cycling efficiency and is safe.